Exhaust mufflers are well known in the art, and are used with any number of conventional combustion engines. The typical exhaust muffler is a spun-head, wrapped muffler generally formed in a cylindrical shape. The muffler shell is wrapped around a number of internal baffles and the shell is bounded on either end of the muffler by metal plates or heads. The muffler usually takes the shape of a circular cylinder or an elliptical/oval cylinder. The wrapped shell is conventionally comprised of either a single layer of metal or a bi-layer metal laminate.
Every engine while firing produces a range of resonant frequencies up to a maximum resonant frequency. These resonant frequencies are in turn transferred to the muffler. The maximum resonant frequency of the engine, if greater than the natural resonant frequency inherent in any particular muffler design, can lead to the muffler vibrating and producing “shell ring” when the resonant frequency of the engine matches the natural frequency of the muffler. This shell ring can damage the muffler and contribute to environmental noise pollution.
In circular cylindrical mufflers, shell ring is generally not an issue since the geometry of the circle lends structural strength to the muffler shell along its surface. This tends to produce higher natural resonant frequencies than the firing frequencies of engines produced by most global mass-production vehicle manufacturers. However, elliptical/oval mufflers do not typically share in the inherent geometric stiffness of the circle. The natural resonant frequencies of these mufflers tend to fall within the range of engine firing frequencies, making the muffler susceptible to resonating. Elliptical/oval mufflers with single-wall shells are particularly susceptible to shell ring since they inherently posses a low natural resonant frequency. However, use of a bi-layer shell lowers the sensitivity of the shell resonant frequency to the engine firing frequency, allowing for the use of flat-sided elliptical/oval mufflers.
While bi-layer laminated shells possess superior vibration resistance, they have some limitations. Typical bi-layer laminated shells are prone to short lives, anywhere from 3 to 4 years compared with the 5 to 7 year life-span of a single-wall shell. This shortened life-span for bi-layer laminated shells is due to accelerated corrosion. Water vapor condenses in the spaces between the lamina since these spaces are not able to vent to the atmosphere. This water entrapment accelerates the corrosion of the shell, causing earlier failure. The single-wall shell has no spaces in which vapor can condense and is therefore not susceptible to accelerated corrosion.
There remains a need in the art for an elliptical/oval exhaust muffler that combines the longevity of the single-wall shell with the resistance to shell ring of the bi-layer laminated shell. Accordingly, it is an object of the present invention to provide an elliptical/oval exhaust muffler having a single-wall shell with a natural resonant frequency greater than the maximum resonant frequency of an engine.